Update with AddToSet not updating null value with MongoDB C#

Question

Using MongoDB, I'm having trouble adding en element to an Array when the array is null. AddToSet works as expected if I add the item from the console. I am using the official C# driver from 10gen.

var query = Query.EQ("_id", objectId);          
var itemDoc = item.ToBsonDocument();

//items is an array but currently null
var update = MongoDB.Driver.Builders.Update.AddToSet("items", itemDoc); // YUNoWork?

//somefield doesn't exist
var workingUpdate = MongoDB.Driver.Builders.Update.AddToSet("somefield", itemDoc); //works fine

var collection = DataBase.GetCollection<MyObject>(CollectionName);

collection.Update(query, update); // doesn't work
collection.Update(query, workingUpdate); // works

Is this expected behavior? If so, is there a more general way to add items to an array?